Dental amalgam has been available to the dental profession for well over a century and it is used extensively for intracoronal and extracoronal restorations. Amalgam is highly durable and the strength and occlusal wear characteristics of alternative materials such as composite resins are generally compared to that of amalgam. However, amalgam does not adhere to tooth structure and the dentist must take great care to prepare the tooth cavity with dovetails and various cutout grooves which in effect mechanically lock the amalgam into the cavity. Such required preparation by the dentist results in the need to excavate more tooth structure than would otherwise be necessary if the amalgam were adhesive. This of course weakens the tooth. Additionally, microleakage tends to occur at the interface of the amalgam and cavity wall. Microleakage allows penetration of bacteria, soluble salts and saliva into any space between the amalgam restoration and cavity walls. This can lead to inflammation and pulp irritation which in turn can cause other complications. The penetration of bacteria into spaces between the amalgam and cavity wall can demineralize the cavity walls and lead to formation of recurrent caries. Corrosion of amalgam can cause one of the amalgam alloy metals, for example tin, to deposit along the amalgam and cavity wall interface. This usually leads to tooth discoloration along the interface and can noticeably detract from the esthetic appearance of tooth and restoration. An adhesive seal between amalgam restoration and cavity walls could prevent microleakage. An adhesive amalgam could significantly reduce the amount of tooth the dentist needs to excavate in order to prepare the cavity for restoration. An adhesive amalgam could impart significantly more strength to the filled tooth structure.
U.S. Pat. No. 3,513,123 (Saffir) discloses an epoxy liquid resin composition which is added to amalgam in an effort to make the amalgam adhere to tooth structure. This reference discloses use of an epoxy liquid resin additive consisting of a glycidyl ether type epoxy resin containing a polyamine hardening agent.
U.S. Pat. No. 4,064,629 (Stoner), discloses a method for applying amalgam restorations which involves precoating the surfaces of a cavity within a carious tooth with a layer of an "adhesive-metal" lining composition. The metal of the lining composition is amalgamated by diffusion of the mercury from the subsequently applied conventional dental amalgam filling. The "adhesive-metal" lining composition is said to improve corrosion resistance of the dental amalgam filling and also promotes bonding between the amalgam restoration and the cavity surfaces. Other references which disclose precoating the surfaces of a tooth cavity with an adhesive coating said to adhere to conventional amalgam are, for example, U.S. Pat. Nos. 4,001,483 (Lee) and 3,574,943 (Nicholson).
In recent years several adhesive products which claim to make amalgam adhere to tooth structure have been made available to dental clinicians. (The term "tooth structure" as used hereinafter shall be interpreted to include either or both dentin and enamel.) One such product is sold in a kit form under the trademark "AMALGAMBOND" available from Parkell Co. The "AMALGAMBOND" product is a liquid adhesive resin which is coated directly onto tooth structure. The curing procedure also requires use of an air-sensitive catalyst which if dropped on flammable paper causes smoldering. The active ingredients in the adhesive are 4-META (4-methacryloxyethyl trimellitic anhydride) and TBB (tri-n-butylborane). Other products which similarly involve coating a specific curable resin directly onto tooth structure to make amalgam adhere are available under the trademarks "PANAVIA" Dental Adhesive from Kuraray Company and "SUPERBOND" Adhesive from Sun Medical Co., Ltd., Kyoto, Japan.
Literature articles which disclose bonding of amalgam to tooth structure by precoating the tooth with adhesive resin include M. Staninec and M. Holt, Journal of Prosthetic Dentistry (1988), Vol. 59, p. 397-402, A. Lacey and M. Staninec, Quintessence International (1989), Vol. 20, p. 521-524, and Y. Torii, et al. Operative Dentistry (1989), Vol. 14, p. 142-148. The above listed articles report improved adhesive tensile strength between amalgam and coated tooth structure but do not report adhesive shear bond strength of the amalgam.
U.S. Pat. No. 5,276,068 to Waknine discloses dental compositions useful for bonding dental surfaces, including enamel, dentin, porcelain and metallic surfaces, comprising polycarbonate dimethacrylate condensation products as a principle component, and a secondary monomer such as BIS-GMA or urethane dimethacrylate or the like as a second component, which is provided to impart strength to the dental composition. Also described therein are methods for bonding dental restorative materials to an exposed dentin surface, wherein the surface can be pretreated by application of 3% H.sub.2 O.sub.2, 17% EDTA, or 5% NaOCl in non-vital teeth followed by an alcohol or acetone solution of an alkali metal salt of benzenesulfinic acid with subsequent evaporation of the alcohol from the solution. Alternatively, the surface can be pretreated by first applying an alcohol or acetone solution of an alkali metal salt of benzenesulfinic acid and then applying an acetone solution of N-phenylglycine. The treated dentin surface is then coated with a resinous adhesive. The adhesive is then cured and an appropriate dental restorative material is applied.